1. Field of the Invention
The present invention relates to a method for estimating frequency offset of a wireless communication system, and relates to a method for estimating and compensating frequency offset.
2. Description of Related Art
Along with the steady progress in communication technology, carrying more data bits in a limited bandwidth is an inevitable trend. In order to more effectively utilize spectrum, a communication technique using multi-carriers has been developed, e.g. orthogonal frequency division multiplexing (OFDM). An OFDM system occupying a limited bandwidth composes of a plurality of sub-carriers, wherein a plurality of sub-carriers are used for parallel signal transmissions, orthogonality is sustained between the sub-carriers and each sub-carrier carries a different amount of data bits by using various modulation schemes.
An OFDM system is predominant in high transmission rate and robustness against multi-paths' effects. Undoubtedly, OFDM is one of important techniques in wireless transmission field. So called worldwide interoperability for microwave access (WiMAX) set forth not long ago is just based on the OFDM architecture following along the IEEE (Institute of Electrical and Electronics Engineers (IEEE)) standards 802.16 and 802.16a-802.16e. In addition, a new system architecture of orthogonal frequency division multiple access (OFDMA) capable of supporting multiple subscribers has been evolved from the OFDM technique by the IEEE standards 802.16d and 802.16e.
Recently, OFDM and OFDMA are broadly used in communication field. In an OFDM system, a transmission terminal groups all sub-carriers into multiple segments and performs an inverse fast Fourier transform (IFFT) to convert symbols on these segments from frequency domain into time domain as transmitted signals, and thereafter, a reception terminal performs a fast Fourier transform (FFT) to convert the transmitted signals from time domain back into frequency domain as received symbols. Accordingly, signal synchronization becomes the crucial task for an OFDM system. In other words, the IFFT/FFT operations are largely affected without the signal synchronization, which even breaks the orthogonality between sub-carriers and hereby causes inter carrier interference (ICI). Moreover, frequency offsets of the oscillators at the transmission terminal and the reception terminal thereof and the Doppler shift effect produced by reception terminal movement also break the orthogonality between the sub-carriers and increase interference. Therefore, to effectively operate an OFDM system, the reception terminal thereof needs to estimate and compensate the carrier frequency offset (CFO).
FIG. 1A is a diagram of a conventional preamble structure on time domain for an orthogonal frequency division multiple access (OFDMA) and FIG. 1B is a waveform diagram in accordance with the conventional preamble of FIG. 1A. Referring to FIGS. 1A and 1B, in a downlink system, a preamble 10 contains three quasi-periods 101, 103 and 105 according to IEEE 802.16e, wherein the preamble 10 is served for frequency synchronization.
It should be noted that the size of a discrete Fourier transform (DFT) is a power of 2, which is unable to be divided by an odd number exactly. Thus, the quasi-periods 101, 103 and 105 are not identical although the quasi-periods 101, 103 and 105 under the ideal condition are identical to each other, for example, the ideal waveform in FIG. 1B. Once a preamble 10 with a serious quasi-periodic property distortion is used to estimate frequency, a quite poor estimation is expected.
In order to improve the quasi-periodic structure of the preamble 10 (almost-periodic preamble), a scheme by increasing the sampling frequency is provided. FIG. 1C is a waveform diagram of, for example, a conventional preamble after increasing the sampling frequency thereof. Referring to FIGS. 1B and 1C, the real sampled signal waveform shown by FIG. 1C indicates a case where the sampling frequency is double that in FIG. 1B. It can be seen that the quasi-periods 101, 103 and 105 in FIG. 1C still contain serious mismatches although the sampling frequency of the preamble 10 is doubled. When the sampling frequency approaches infinity, the quasi-periodic property of the preamble 10 can be theoretically improved but the cost of the hardware is high, which is impractical. Therefore, the manufactures are looking for an alternative solution to reconstruct a better quasi-periodic property of the preamble 10, which is not only cost effective but also increases the accuracy of estimating a carrier frequency offset for an accurate compensation.